The exhaust gas produced by the combustion of air and fuel in an internal combustion engine includes regulated constituents such as carbon monoxide (“CO”), unburned hydrocarbons (“HC”) oxides of nitrogen (“NOx”) and, in the case of diesel engines, condensed phase materials (liquids and solids) which constitute particulate matter. Manufacturers of internal combustion engines are increasingly focused on the development of engine control strategies that satisfy both customer demands for performance as well as various government regulations for exhaust gas emissions and fuel economy. One such engine control strategy comprises operating an internal combustion engine at an air/fuel ratio that is lean of stoichiometry to improve fuel economy and reduce greenhouse gas emissions. Such operation is possible using both compression-ignition (diesel) and lean-burn spark-ignition (gasoline) engines. When an engine operates with a lean (excess oxygen) air/fuel ratio, the exhaust gas may contain higher levels of engine-out NOx emissions. Commercial application, especially automotive application of lean operating engines, has been limited due to a lack of effective methods for the removal of NOx under a lean exhaust condition. As such, the efficient reduction of NOx (NOx=NO+NO2) from diesel and lean-burn gasoline exhaust gas is important to meet future emission standards and improve fuel economy.
Reduction of NOx emissions from an exhaust gas feed stream containing excess oxygen is a challenge for vehicle manufacturers. Several potential aftertreatment systems have been proposed for vehicle applications. One approach comprises using an aftertreatment system that includes injecting a NOx reductant (e.g. aqueous urea), upstream of a urea-SCR catalyst, to reduce NOx to N2. The use of urea as a reductant necessitates the implementation of a urea storage and distribution system on board the vehicle. Part of such a storage and distribution system comprises an apparatus for effective introduction of the urea into the exhaust gas feed stream in a manner that allows for evaporation and mixing of the urea with the exhaust gas. The urea requires adequate mixing and residence time in the hot exhaust gas stream to decompose to produce ammonia (“NH3”) as a reaction by-product, since it is the NH3 that is used as a reactant species in the catalytic reactions that occur in the urea-SCR catalyst device. Introduction of the urea into the exhaust gas feed stream may be through an injection device, similar to a fuel injector, that is in fluid communication with a source of liquid urea. Because urea has a latent heat of vaporization which is significantly greater than that of fuel, for example, additional devices may be placed into the exhaust gas feed stream, downstream of the injection site, to assist in evaporation and mixing. One such device is an impaction plate or nozzle diffuser that is configured to capture larger droplets of the injected urea on its surface for eventual evaporation caused by exposure to the exhaust gas flow. While such nozzle diffusers have met with some success, the vaporization rate of the urea has varied from 50 to 70%. Such a rate results in higher than desired consumption of the urea reactant as well as deposition of liquid urea on the downstream urea-SCR catalyst device which may degrade its performance.